I wouldn't assume anything! If it is a shop that only sells reptiles (not fish, puppies, etc), then it is more likely they know what they are doing, compared to a regular pet shop. But it is not guaranteed.
Stick your nose down into the bag and smell them. If they have a strong smell like pine or cedar, I would use on rodents or take it back. If it has no smell, or just an earthy smell (like potting soil), then it is PROBABLY ok. But you might have to ask the shop or go online if the bag has a website on it, so you can tell for sure.